This application claims the priority of German Patent Application No. 198 12 171.7, filed Mar. 19, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for operating a motor vehicle air conditioner with a compressor lubricated by circulated lubricant.
It is known that so-called externally regulated compressors can be used in motor vehicle air conditioners. The output of such compressors can be controlled externally in stages or varied continuously in order to set the air conditioner output to a desired value. In conventional air conditioners without the ability to regulate the output of the compressor, the air conditioner output is set by connecting and disconnecting the compressor cyclically in suitable fashion by a magnetic clutch or the like, in other words to a mechanical drive such as the vehicle engine. Such externally regulated compressors and methods for their control are described for example in German Patent DE 38 43 924 C2 and German Patent Application No. 196 42 832.7.
Such externally regulated compressors are also frequently lubricated with a circulated lubricant in the form of a suitable lubricating oil that is added to the coolant and is circulated with the coolant in the coolant circuit. A sufficient lubricant supply is particularly important for externally regulated compressors that are connected without clutches to mechanical drives because they have mechanically movable parts turning constantly even without any demand for compressor output and at low outside temperatures.
It is also known that each time the cooling operation of motor vehicle air conditioners begins, there is a risk of so-called "flash fog" formation. When the system starts, the air stream guided over the still-wet evaporator surface becomes humidified and results in fog forming on the interiors of vehicle windows subjected to the flow. To avoid this effect, it is known that the air conditioner can be started in so-called reheat operation each time, in other words dehumidification operation. The coolant circuit is set for maximum cooling output, and the dry air cooled in this way is then heated suitably by a heating body. Depending on the actual cooling output requirement, after a certain amount of time, a gradual transition is made from reheat operation to the actually desired operating mode in systems with so-called internally regulated compressors (in other words those that are activated and deactivated by a clutch), wherein the compressor, which is constantly connected during reheat operation, is disconnected and connected alternately. The time intervals during which the compressor is disconnected is lengthened suitably.
The technical problem to be solved is the provision of a method in which a motor vehicle air conditioner that has a compressor lubricated by a circulated lubricant can be operated so that a lubricant supply is guaranteed that is always sufficient for the compressor.
The present invention solves this problem according to the following methods.
In a method according to the present invention, the compressor, after starting, is kept in the lubricant-circulating operating mode for at least a specified minimum lubrication time. By suitably specifying this minimum lubrication time, compressor operating times that are too short, which could result in an insufficient supply of lubricant for the compressor, are avoided. On the other hand, in a corresponding operating situation, maintenance of lubricant-circulating compressor operation for the minimum lubrication time can be combined after each new start with an initial reheating operation of the system to avoid the flash-fog effect. As a result of maintaining lubricant-circulating compressor operation for a suitably specified minimum lubricating time in the case of a lubricant that is added to the coolant as an additive, the lubricant is able to supply a sufficient quantity to lubricate the compressor, following a new start of the system and after being circulated in the coolant circuit.
In a method according to another embodiment of the present invention, the minimum lubrication time is advantageously specified to vary as a function of the outside temperature. Shorter minimum lubrication times are selected for higher outside temperatures than for lower outside temperatures. Since more water of condensation is formed during reheating operation with increasing outside temperature, the shorter minimum lubrication time for higher outside temperatures prevents the formation of quantities of water of condensation that are undesirably high.
In a method according to another embodiment of the present invention, the compressor is operated at maximum output at least during an initial part of the minimum lubrication time, with the flash-fog effect being counteracted. The compressor output is then gradually adjusted to a value required by the user. This gradual transition takes place in another embodiment of the present invention with a PT1 time response like that which can be achieved by using a conventional PT1 element.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.